Various seal devices are known for forming a seal between a rotatable shaft, or a sleeve or runner mounted on a rotatable shaft, and a housing or other structure surrounding the shaft. One type of seal, sometimes referred to as a contact circumferential shaft seal, is very effective in controlling leakage. Such seals include one or more seal rings with circumferential inner faces that contact the rotating shaft and slide against the shaft while it rotates. These seals may be formed from compacted and sintered carbon graphite to provide heat and wear resistance, and they are often formed as a plurality of interconnectable ring segments to facilitate installation about the shaft. The seal rings are held in place by a suitable retaining device and may include a biasing device, such as a circumferential or garter spring, for holding the seal segments together.
While carbon seal rings are durable and capable of withstanding high levels of heat and friction, sliding contact with a rotating shaft eventually causes the rings to wear out. The wear rate is accelerated as the seal gas pressure is increased. The desire for longer operating life and higher thermal efficiency has therefore moved the seal industry to look for alternatives to circumferential contact seals.
One conventional alternative to contact seals are solid seals that are always spaced a small distance from the rotating shaft. These seals never contact the moving shaft and therefore do not wear like contact seals. However, because there is always a small gap between the seal and the shaft, a relatively large amount of leakage through the seal occurs, especially at high pressures.
Another alternative to circumferential contact seals is a circumferential gas film seal. Much like the circumferential contact seal, this seal includes one or more carbon seal rings that exert a very light contact force against the shaft when it is not rotating. The seal ring lifts-off the shaft or sleeve when the shaft is rotating. The lift-off is achieved by routing high pressure gas to opposing faces of the seal through clearance spaces and milled cutouts. In the case of a contacting circumferential seal, the outer diameter of the ring is exposed across its entire width while the inside diameter is exposed across its entire width excluding the width of a small sealing dam across which there is a pressure breakdown. This creates an imbalance in force that causes the seal to stay in contact with the shaft. Producing a force balanced contact in this manner is referred to hydrostatic sealing, and a hydrostatic seal can be maintained both when the shaft is rotating and when the shaft is stationary. Alternately or in addition, hydrodynamic sealing can be produced by forming recesses or cutouts on the side of the seal ring that faces the shaft. As the shaft rotates, gas entrained by the rotating shaft is compressed in these cutouts, and as the gas escapes over the non-recessed “pads” between the recesses, it produces an additional pressure and flow of gas for maintaining a separation between the seal ring and the shaft. Circumferential gas film seals generate less friction and less heat than circumferential contact seals, and thus generally last longer, require less maintenance and less external cooling than contact seals.
Circumferential gas film seals are useful in environments where a small amount of gas passes continuously between the seal and the shaft which leaking gas provides the hydrodynamic balancing discussed above. Steam turbines include rotating shafts that required sealing and could benefit from circumferential gas film seals. However, such seals have heretofore not been used for steam turbines. This is partly because the material passing between the seal and the rotating shaft is not always a gas. At operating speeds and temperatures, the material passing between the shaft and seal is high temperature steam. However, under certain other operating conditions, including at startup, the steam may be in the condensed liquid form, and the presence of this liquid or condensate around the seal may prevent or significantly interfere with optimal or successful non-contacting seal performance as discussed above. It would therefore be desirable to provide a circumferential seal that can operate efficiently under the conditions present at the shaft/seal interface of a steam turbine or in other environments in which the material passing between the seal and shaft is not always entirely in a gaseous state.